The present invention relates to burner ignition circuits for producing electrical sparks to ignite the fuel of a fuel oil burner or the like and, more particularly, to such circuits which produce ignition sparks at a frequency substantially in excess of the AC power supply therefor.
As discussed in U.S. Pat. No. 3,556,706, conventional fuel oil burners or the like include a nozzle for creating a spray pattern of oil particles in an air stream produced by a blower, which have been traditionally ignited as they emerge from the nozzle by sparks created between a pair of spark electrodes located upstream in the air stream from the spray pattern powered by high voltage step-up transformers coupled to an AC supply. More recently, electronic ignition circuits, such as the one shown in U.S. Pat. No. 3,556,706, have been provided which produce the requisite high frequency ignition sparks comparable to those produced by the aforementioned high voltage step-up transformers, but are smaller in size, lighter in weight, less expensive and more efficient than the conventional spark transformers.
While it may be suggested that the ignition systems such as the one in the aforementioned patent function in a more or less satisfactory manner, certain apparent disadvantages exist in such circuits. For example, although an AC power supply is utilized, the circuit is operative to produce high frequency sparking only during the positive half waves of the power supply. In addition, the SCR switch used to discharge the capacitor is slowly turned on by a long transition trigger signal developed by an RC circuit. The relatively long turn-on time of the SCR results in an output signal having a magnitude less than that which would otherwise be produced.
A further problem encountered in circuits such as the one shown in the aforementioned patent is that at the end of each discharge cycle, oscillating residual energy stored in the LC circuit formed by the primary winding and the capacitor may result in development of a reverse polarity charge on the capacitor that must be overcome during the next charging cycle. Further, the undesirable oscillating residual energy may result in partial cancellation of the next discharge current signal through the primary winding. An attempted solution to this problem has included the provision of a feedback circuit including a diode connected between the primary winding and the discharge capacitor to return this undesired residual energy back to the capacitor to charge it in the desired polarity direction. However, because of the manner in which such circuits have been connected back to the capacitor, LC filter circuits using expensive circuit elements have had to be included in the feedback network.